1. Field of the Invention
The present invention relates to a control device for a cylinder injection internal-combustion engine in which fuel is directly injected into a cylinder and, more particularly, to a control device for a cylinder injection internal-combustion engine with improved combustion efficiency of the engine in a compression stroke injection mode.
2. Description of Related Art
FIG. 27 is a block diagram showing the entire system of a control device of a typical cylinder injection internal-combustion engine.
The system shown in the drawing includes: an engine 1 which provides the main body of the internal-combustion engine and which is composed of a plurality of cylinders 1a through 1d; an inlet pipe 2 which supplies air to the cylinders 1a through 1d of the engine 1; an air cleaner 3 provided at the inlet port of the inlet pipe 2; a throttle valve 4 which is installed in the inlet pipe 2 and which adjusts an inlet air amount Q; and a surge tank 5 provided in the intake manifold of the inlet pipe 2.
The system further includes: a throttle valve lift sensor 6 which detects lift .theta. of the throttle valve 4; a throttle valve actuator 7 which opens and closes the throttle valve 4; a fuel injection valve 8 which directly injects fuel into the cylinders 1a through 1d; an ignition coil unit 9 provided in each of the cylinders 1a through 1d; and a spark plug 10 driven by high voltage applied by the ignition coil unit 9.
Further included in the system are: an accelerator pedal 11 operated by a driver who steps thereon; an accelerator depression sensor 12 which detects the amount of depression .alpha. of the accelerator pedal 11; a crank angle sensor 13 which is provided on a crankshaft of the engine 1 and which issues a crank angle signal SGT; a cylinder identifying sensor 14 which is provided on a cam shaft interlocked with the crankshaft and which issues a cylinder identification signal SGC; an oxygen concentration sensor 15 which detects the oxygen concentration X in the exhaust gas discharged from the engine 1; and a catalyst 16 which purifies the exhaust gas.
The sensors 6 and 13 through 15 constitute the diverse sensors for outputting operational information. Other sensors such as an airflow sensor and an inlet pipe pressure sensor for detecting the inlet air amount Q are also provided although they are not shown.
Also included in the system are: an intracylindrical pressure detecting unit 17 which detects pressure P in cylinders 1a through 1d of the engine 1 (hereinafter referred to as "intracylindrical pressure"); a knocking sensor 18 which detects knocking vibration K of the engine 1; and an ionic current detecting unit 19 which detects an ionic current C indicative of the combustion degree in the cylinders 1a through 1d.
An electronic control unit 20 is comprised of a microcomputer; it computes diverse types of control amounts according to the operational information .theta., SGT, SGC, X, K, P, and C received from the various sensors 6, 13 through 15, and 18, and the detecting units 17 and 19 so as to control the engine 1 according to control signals J, G, and R based on the computed control amounts.
For instance, the electronic control unit 20 computes the target lift of the throttle valve 4 from the depression amount .alpha. of the accelerator pedal 11, and controls the throttle valve actuator 7 according to a lift control signal R, thereby conducting feedback control so that the lift .theta. of the throttle valve 4 coincides with the target lift.
The electronic control unit 20 computes engine speed Ne from the crank angle signal SGT, computes a target engine torque from the engine speed Ne and the depression amount .alpha. of the accelerator, computes a target fuel injection amount Fo from the engine speed Ne and the target engine torque To, and drives the fuel injection valve 8 according to the injection signal J of a driving time based on the target fuel injection amount Fo.
The electronic control unit 20 computes the ignition timings for the cylinders 1a through 1d mainly according to the crank angle signal SGT and the cylinder identification signal SGC, and causes the spark plug 10 to discharge by driving the ignition coil unit 9 in accordance with the ignition signal G.
Furthermore, the electronic control unit 20 detects the occurrence of knocking according to the knocking vibration K, and if knocking occurs, then it delays the ignition signal G to restrain the knocking.
The electronic control unit 20 also determines the combustion state of the cylinders 1a through 1d or detects the occurrence of a misfire primarily according to the intracylindrical pressure P and the ionic current C.
FIG. 28 is a block diagram detailedly showing the specific configuration of the electronic control unit 20 shown in FIG. 27.
The electronic control unit 20 shown in FIG. 28 includes: a microcomputer 21; input interfaces (I/Fs) 22 and 23 which take various types of operational information into the microcomputer 21; a power circuit 24 which supplies electric power to the microcomputer 21; and an output I/F 25 which outputs the control signals R, J, and G received from the microcomputer 21. An ignition switch 27 connects an on-car battery 26 to the electronic control unit 20 at the time of startup.
The microcomputer 21 is equipped with: a CPU 31 which mainly controls the fuel injection valve 8 and the spark plug 9 according to a predetermined program; a free-running counter 32 for detecting the rotational cycle from the crank angle signal SGT; a timer 33 for measuring time for performing diverse types of control; and an analog-to-digital converter 34 for converting an analog signal received from the input I/F 23 to a digital signal; a RAM 35 used as the work area of the CPU 31; a ROM 36 wherein an operating program for the CPU 31 has been stored; an output port 37 through which various driving control signals such as J, R, and G are output; and a common path 38 for connecting the CPU 31 with the constituent elements 32 through 37.
The input I/F 22 shapes the waveforms of the crank angle signal SGT and the cylinder identification signal SGC and supplies the shaped waveforms to the microcomputer 21 as interrupt signals. When an interrupt signal is received from the input I/F 22, the CPU 31 in the microcomputer 21 reads the value on the counter 32, computes the pulse cycle of the crank angle signal SGT from the difference between the present value and the previous value, and stores it in the RAM 35 as the value corresponding to the current engine speed Ne.
The CPU 31 also detects, at the time of the interrupt, the signal level of the cylinder identification signal SGC to detect which of a plurality of the cylinders 1a through 1d corresponds to the crank angle signal SGT detected this time.
The input I/F 23 supplies the detection signals such as the throttle valve lift .theta., the intracylindrical pressure P, accelerator depression amount .alpha., and oxygen concentration x to the CPU 31 in the microcomputer 21 via the analog-to-digital converter 34.
The output I/F 25 amplifies the diverse control signals issued from the CPU 31 via the output port 37 and supplies them to the throttle valve actuator 7, the fuel injection valve 8, the ignition coil unit 9, etc.
FIG. 29A through FIG. 29D show timing charts illustrative of the control timings of the injection signal J and the ignition signal G generated by the electronic control unit 20; it illustrates the relationship between the pulse waveforms of the cylinder identification signal SGC and the crank angle signal SGT, the fuel injection timing of the fuel injection valve 8, and the driving current of the ignition coil unit 9.
FIG. 29A shows the pulse waveform of the cylinder identification signal SGC; FIG. 29B shows the pulse waveform of the crank angle signal SGT; FIG. 29C shows the injection signal J for the fuel injection valves 8 of cylinders #1 through #4; and FIG. 29D shows ignition signal G for the ignition coil units 9 of cylinders #1 through #4.
Each pulse of the crank angle signal SGT rises, for example, at 75 degrees before reaching the top dead center (B75 degrees) corresponding to the initial energizing start timing of each cylinder, and falls at 5 degrees before reaching TDC (B5 degrees) corresponding to the initial ignition timing of each cylinder.
The cylinder identification signal SGC is issued during the compression stroke of cylinder #1 of the engine 1. Once the electronic control unit 20 recognizes the pulse of the crank angle signal SGT that corresponds to cylinder #1, it is able to tell which pulses of the crank angle signal SGT correspond to cylinders #1 through #4 of the engine 1.
Since the rising edge of the crank angle signal SGT indicates B75 degrees of a corresponding cylinder and the falling edge indicates B5 degrees of the corresponding cylinder, the electronic control unit 20 detects those edges indicative of B75 degrees and B5 degrees by the interrupt function of the microcomputer 21 to use them as the reference positions of the fuel injection timing and the ignition timing.
In the case of the cylinder injection internalcombustion engine, the combustion state of the engine 1 depends on the falling timing, i.e. the fuel injection end timing, of the injection signal J and the falling timing of the ignition signal G, i.e. the ignition timing.
Normally, when the fuel injection end timing and the ignition timing have been decided to ensure optimum fuel consumption rate, the fuel injection end timing is controlled so that it is slightly delayed from the rising edge B75 degrees (e.g. approximately B60 degrees) of the crank angle signal SGT, while the ignition timing is controlled so that it is slightly advanced (approximately B15 degrees) from the falling edge B5 degrees of the crank angle signal SGT.
The CPU 31 in the electronic control unit 20 determines to which cylinder the crank angle signal SGT corresponds in accordance with the cylinder identification signal SGC, and applies the injection signal J matched to the fuel injection timing so as to inject the predetermined amount Fo of fuel to the fuel injection valve 8 of the cylinder under the control.
The CPU 31 also issues the ignition signal G matched to the ignition timing to the ignition coil unit 9 of the cylinder under the control. This causes the ignition coil unit 9 to apply the high voltage obtained by amplifying battery voltage to the spark plug 10 to ignite and burn the fuel at the computed control timing.
Thus, the fuel is directly injected into the cylinders 1a through 1d, and the injected fuel burns to run the engine 1.
The specific operation of the control device of a conventional cylinder injection internal-combustion engine configured as shown in FIG. 27 and FIG. 28 will now be described with reference to the timing charts of FIG. 29A through FIG. 29D and the schematic representations and the characteristic charts of FIG. 30 through FIG. 36.
FIG. 30 illustrates the relationship between the fuel injection mode and the engine speed Ne and the target engine torque To. The hatched area wherein the target engine torque To is ToA or less and the engine speed Ne is NeB or less indicates that the engine 1 consumes a smaller amount of fuel per cycle.
Hence, in the aforesaid area, the driving time, i.e. the pulse width of the injection signal J, of the fuel injection valve 8 can be set to a smaller value, and the compression stroke injection mode in which the fuel is injected during the compression stroke of the engine 1 is implemented. In the compression stroke injection mode, the combustion takes place locally in the cylinders. 1a to 1d, namely, in the vicinity of the spark plugs 10, requiring less fuel relative to a cylinder volume. This provides an advantage in that better economy and easier control of the air/fuel ratio for combustion can be achieved.
FIG. 31 is a characteristic chart illustrative of the relationship between the air/fuel ratio (A/F) and the engine-generated torque Te; the solid line denotes the characteristic curve observed in the compression stroke injection mode, and the chain line denotes the characteristic curve observed in the suction stroke injection mode.
As is obvious from FIG. 31, the compression stroke injection enables the engine-generated torque Te to be controlled according to the air/fuel ratio A/F even when the stoichiometric air/fuel ratio (14.7) is set to a value for a leaner mixture.
Conversely, in FIG. 30, when the target engine torque To exceeds ToA or when the engine speed Ne exceeds NeB, the injection of the predetermined fuel amount Fo cannot be completed in the compression stroke. For this reason, the suction stroke injection is performed so that the fuel is injected during the period from the suction stroke to the compression stroke. A comparative reference values ToA and NeB may be fixed values which have been preset as necessary or arbitrary variables.
In the suction stroke injection mode, the same fuel injection and combustion state as those observed in an engine, not shown, wherein fuel is injected in the vicinity of the inlet port will be obtained, so that combustion is implemented by using all the cylinder volume, providing an advantage of a higher engine output.
FIG. 32 and FIG. 33 are schematic representatives illustrative of the combustion states generated by the different fuel injection modes mentioned above; FIG. 32 schematically shows the combustion state observed in the compression stroke injection mode, and FIG. 33 schematically shows the combustion state observed in the suction stroke injection mode.
The schematic representatives show a combustion chamber 40 in a cylinder of the engine 1, an intake valve 41 which communicates the combustion chamber 40 to the surge tank 5, an exhaust valve 42 which communicates the combustion chamber 40 to an exhaust pipe, a combustion area 50 wherein combustion takes place in the compression stroke injection mode, and a combustion area 51 wherein combustion takes place in the suction stroke injection mode.
As shown in FIG. 32, in the compression stroke injection mode, a small amount of fuel is injected into the combustion chamber 40, the fuel is gathered in the vicinity of the spark plug 10, then combustion takes place only in the area around the spark plug 10 in the form of a layer of a concentrated mixture (see the combustion area 50).
At this time, even when the same inlet air amount Q of the engine 1 is used, the generated torque Te of the engine 1 changes depending on the amount of fuel injected in the vicinity of the spark plug 10; hence, the fuel injection amount Fo is changed according to the target engine torque To.
In the suction stroke injection mode, the fuel is injected during the suction stroke and dispersed in the entire area inside a cylinder, so that the combustion takes place in the entire area inside the cylinder as shown in FIG. 33 (see the combustion area 51).
Generally, if the fuel injection amount Fo is increased, whereas the air/fuel ratio A/F is set in the vicinity of the stoichiometric air/fuel ratio (14.7) which enables combustion, then the suction stroke injection mode shown in FIG. 33 is employed because the injection of the fuel cannot be completed during the compression stroke and the fuel cannot be sufficiently dispersed in a cylinder in the compression stroke injection mode.
In the compression stroke injection mode shown in FIG. 32, the fuel injection timing based on the injection signal J and the ignition timing based on the ignition signal G greatly influence the combustion efficiency; if the time from fuel injection to ignition is too short, then the fuel does not reach the area near the spark plug 10 at the time of ignition, preventing optimum combustion from taking place.
Conversely, if the time from fuel injection to ignition is too long, then the fuel is ignited after passing the spark plug 10, also preventing optimum combustion from taking place.
Thus, appropriate fuel injection timing and ignition timing are determined as described below although they vary according to parameters such as the engine speed Ne and the target engine torque To.
FIG. 34 through FIG. 36 are characteristic charts illustrative of the combustion efficiency of the engine 1 when the fuel injection timing, i.e. the injection end timing, and the ignition timing are changed under a certain operating condition; the axis of abscissa indicates the injection end timing, i.e. the position based on the crank angle, the axis of ordinate indicates the ignition timing, i.e. the position based on the crank angle, and W denotes the point at which the fuel consumption rate is the highest (e.g. the injection end timing is B60 degrees and the ignition timing is B15 degrees).
FIG. 34 shows the increase and decrease in the exhaust amount of THC such as HC gas in relation to the injection end timing and the ignition timing; the curves indicate the transition of the exhaust amount of THC. In FIG. 34, the exhaust amount of THC is the smallest in the area enclosed by curve a at the bottom center; the exhaust amount of THC increases as the injection end timing and the ignition timing shift from the area enclosed by curve a into the areas enclosed by the outer curves.
FIG. 35 shows the increase and decrease in the frequency of misfires in relation to the injection end timing and the ignition timing. In FIG. 35, the frequency of misfires is the lowest in the area on the left of curve b at the center; hence, the frequency of misfires increases as the injection end timing and the ignition timing shift from the area on the left of curve b into the area defined by the curves at the bottom right.
FIG. 36 shows the fuel consumption rate in relation to the injection end timing and the ignition timing; the fuel consumption rate is the highest in the area enclosed by curve c at the center. This means that the fuel consumption rate becomes worse toward the areas defined by the curves away from curve c.
The fuel injection timing and the ignition timing are determined, taking the combustion efficiency of the engine 1 described above into account. The determining condition is, for example, that the THC exhaust amount and the frequency of misfires are predetermined values or less and the fuel consumption rate is the maximum point W.
Generally, the combustion in the suction stroke injection mode shown in FIG. 33 uses the entire interior space of a cylinder as previously described; hence, the combustion efficiency of the engine 1 is affected less by the fuel injection timing.
In the combustion carried out in the compression stroke injection mode shown in FIG. 32, however, both fuel injection timing and ignition timing can be the factors affecting the combustion efficiency of the engine 1.
Thus, in the compression stroke injection mode, the combustion takes place only in the layer of the concentrated mixture near the spark plug 10; however, not all fuel burns completely. Hence, the mixture at the central portion of the mixture layer is rich and exhibits good combustion, whereas the mixture at the outer peripheral portion of the mixture layer is lean and may fail to burn completely or may not burn at all.
Such incomplete combustion components or unburnt components will be discharged through an exhaust port to open air or remain in cylinders 1a through 1d and adhere to pistons or the spark plugs 10. This means that some fuel components are apt to adhere to the pistons or the spark plugs 10 in the compression stroke injection mode.
As more incomplete combustion components or unburnt components adhere to the spark plugs 10, the insulation resistance of the spark plugs 10 deteriorates, preventing proper sparking from the central electrodes of the spark plugs 10 to the grounding electrodes. As a result, a part of all spark is easily attracted to a portion where the resistance is lower than that of the grounding electrodes of the spark plugs 10.
Thus, when the insulation resistance of the spark plugs 10 lowers, the ignition energy thereof accordingly decreases, leading to the occurrence of a misfire due to a fuel ignition failure.
In addition, as more misfires of the engine 1 occur, unburnt gas is discharged directly to the open air, deteriorating the exhaust gas components, and the burning energy of the fuel also deteriorates with a consequent decrease in the output torque of the engine 1, so that the rotational torque in the engine 1 fluctuates, resulting in deteriorated drivability.
Thus, the conventional control device for a cylinder injection internal-combustion engine has a shortcoming in that incomplete combustion may take place in the outer peripheral portion of the mixture layer near the spark plugs 10 and incompletely burnt components or unburnt components may adhere to the pistons or the spark plugs 10 in the cylinders 1a through 1d.
There has been a problem in that the deteriorated insulation resistance due to the incompletely burnt components or unburnt components adhering to the spark plugs 10 causes a part or all of the sparks from the central electrodes of the spark plugs 10 to the grounding electrodes thereof to be easily drawn to a portion where the resistance is lower than that of the grounding electrodes, and the ignition energy decreases, leading to the occurrence of a misfire.
There has been another problem in that, as the frequency of misfires of the engine 1 increases, the unburnt gas is discharged directly through the exhaust port; hence, the exhaust gas components deteriorate and the burning energy of the fuel decreases. As a result, the output torque of the engine drops and the rotational torque of the engine 1 varies, leading to deteriorated drivability.